1. Field of the Invention
The invention relates generally to swash plate-type compressors. More particularly, the invention relates to swash plate-type compressors having a shoe positioned between a swash plate and a piston.
2. Description of Related Art
Referring to FIG. 6, a known, swash plate-type compressor 1 is depicted. Compressor 1 includes a cylinder block 2, a front housing 3, a cylinder head 4, and drive shaft 5. Cylinder block 2, front housing 3, and cylinder head 4 may be fixably attached by a plurality of bolts 15. A crank chamber 6 may be formed between cylinder block 2 and front housing 3, and drive shaft 5 may be rotatably supported by cylinder block 2 and front housing 3 via a pair of bearings 16a and 16b mounted in front housing 3 and cylinder block 2, respectively. A swash plate 8 may be positioned inside crank chamber 6, and also may be slidably mounted to drive shaft 5. Swash plate 8 may include an arm 81 rotatably connected to an arm 71 of a rotor 7, such that swash plate 8 rotates substantially simultaneously with drive shaft 5. The connection between arm 81 and arm 71 also allows the inclination angle of swash plate 8 to vary relative to drive shaft 5. Moreover, a suction chamber 9 and a discharge chamber 10 may be formed in cylinder head 4, and an electromagnetic clutch 11 for engaging and disengaging drive shaft 5 may be rotatably supported by front housing 3. Further, a drive belt (not shown) may be used to transfer motion from a crankshaft of an engine of a vehicle (not shown) to electromagnetic clutch 11.
Compressor 1 also may include a plurality of cylinder bores 12 formed in cylinder block 2, and a plurality of pistons 13 positioned within a corresponding cylinder bore 12. Cylinder bores 12 may be arranged radially with respect to a central axis of cylinder block 2, and pistons 13 may reciprocate independently within corresponding cylinder bore 12. Each piston 13 also may be connected to swash plate 8 via a pair of shoes 14. Specifically, each shoe 14 may comprise a substantially flat surface and a semispherical portion. The substantially flat surface of shoe 14 may be in slidable contact with swash plate 8, and the semispherical portion of shoe 14 may rotatably engage a semispherical cavity of piston 13. As such, shoes 14 may convert the rotation of swash plate 8 into the reciprocation of pistons 13 within corresponding cylinder bores 12. Specifically, when the inclination angle of swash plate 8 relative to drive shaft 5 varies, shoes 14 may maintain rotational engagement with piston 13 and also may maintain sliding contact with swash plate 8, which may allow pistons 13 to reciprocate within corresponding cylinder bores 12. When each piston 13 reciprocates, corresponding shoes 14 may rotate about their shared center axis within the semispherical cavity of piston 13.
Because of the rotation of shoe 14 within the semispherical cavity of piston 13, a lubricant, e.g., a lubricating oil, may be employed in order to reduce or eliminate friction between shoe 14 and piston 13. In order to more readily supply the lubricant between the engaging portions of shoe 14 and piston 13, the semispherical portion of shoe 14 may have a substantially flat or a convex, semispherical portion formed at a piston-side of shoe 14. The substantially flat or convex, semispherical portion of shoe 14 may have a radius of curvature which is greater than a radius of curvature of a seat portion of the semispherical cavity of piston 13. As such, a gap or a clearance may be created between the substantially flat or convex, semispherical portion of shoe 14 and the semispherical cavity of piston 13. Examples of such known shoes are described in Japanese (Examined) Utility Model Publication No. H07-5259, Japanese (Unexamined) Patent Publication No. H11-50958, and Japanese (Unexamined) Patent Publication No. 2000-170653. Nevertheless, with these known shoes, the substantially flat or convex, semispherical portion formed at the piston-side of the shoe may deform during manufacture of the shoe because of a wear reduction heat treatment applied to the shoe during manufacture. As such, it may be difficult to accurately maintain the shape of the substantially flat or convex, semispherical portion formed at the piston-side of the shoe. Specifically, during manufacture, the perimeter of the substantially flat or convex, semispherical portion formed at the piston-side of the shoe may become a circular-shaped perimeter.
During operation, when the pistons reciprocate within the cylindrical bores, the seat portion of the semispherical cavity of the piston engages the substantially flat or convex, semispherical portion of the shoe. Nevertheless, because the substantially flat or convex portion formed at the piston-side of the shoe has a circular-shaped perimeter, the seat portion of the semispherical cavity of the piston substantially seals the substantially flat or convex portion of the shoe during a rotation of the shoe. As such, the amount of lubricant distributed from the substantially flat or convex portion of the shoe to other portions of the shoe engaging the seat portion of the semispherical cavity of the piston may be reduced. Consequently, friction between the shoe and the piston may increase, and noise associated with such friction also may increase.
Therefore a need has arisen for swash plate-type compressors having shoes which overcome these and other shortcomings of the related art. A technical advantage of the present invention is that a saddle or groove formed at a piston-side of a shoe may have a non-circular-shaped perimeter, e.g., an oval-shaped perimeter. As such, when a seat portion of a semispherical cavity of a piston engages the saddle portion or the groove of the shoe, the piston may not seal the saddle portion or the groove of the shoe during a rotation of the shoe. Consequently, friction between the shoe and the piston may be reduced or eliminated without increasing the size of the gap or the clearance between the shoe and the piston, and noise associated with such friction also may be reduced or eliminated.
According to an embodiment of the present invention, a swash plate-type compressor is described. The compressor comprises a cylinder block having a plurality of cylinder bores formed therethrough, a drive shaft rotatably supported by the cylinder block, and a swash plate rotatably mounted on the drive shaft. The compressor also comprises a plurality of pistons, each of which is positioned within one of the cylinder bores and reciprocates within the cylinder bore. Each of the pistons comprises a substantially semispherical cavity formed at an end of the piston. The compressor further comprises a pair of shoes positioned between each of the pistons and the swash plate. Each shoe comprises a substantially flat surface adapted to be in slidable contact with the swash plate, and a substantially semispherical portion adapted to rotatably engage the semispherical cavity of the piston. Moreover, the semispherical portion of the shoe comprises a saddle portion or a groove having a first curved portion and a non-circular perimeter.
Other objects, features, and advantages of the present invention will be apparent to persons of ordinary skill in the art in view of the following detailed description of the invention and the accompanying drawings.